There is mutual attraction between molecules inside the liquid, which is caused by the polarity of the liquid molecules. The molecules on the surface of the liquid (usually in contact with the gas) have a combined force perpendicular to the liquid surface and pointing to the inside of the liquid due to the gravitational force of the internal molecules. This force is called the cohesion of the liquid. Different liquids have different cohesion, but any liquid has cohesion.
Due to the effect of cohesion, liquids tend to reduce their surface area, so that only the particles that are absolutely necessary to form the surface exist on the surface of the liquid without any extra particles left on the surface. Therefore, when the liquid exists freely, it is spherical, because the same volume of the sphere has the smallest surface area.
